This invention relates to an improved scheme for detecting a synchronizing bit pattern in a data bit stream, and more particularly to an elegant detection scheme for quickly distinguishing between a synchronizing pattern and apparent synchronizing patterns of data bits.
In the systems to which this invention is applicable, a transmitter sends a stream of binary data bits to which it periodically adds synchronizing or framing code bits in a predetermined pattern. The synchronizing bit pattern is repeated in each data frame so that within a data block or frame the code is continuously repetitive at the framing bit locations.
It will be appreciated that data bits will be also transmitted, from time to time, in a sequence or pattern identical to the synchronizing or framing code pattern. The location of the true framing code pattern can be identified by eliminating locations in the data block that do not continuously contain the synchronizing code pattern.
A simple serial search scheme for frame synchronization has been effectively used in many prior art systems. Basically, this method operates as follows:
assume the first bit received begins a frame; PA1 when the candidate frame is received, test the contents of the frame bit locations to see if they contain the designated frame code; and PA1 if so, wait for the next frame, check again, and keep track of the number of times this is repeated, ending with a declaration of synchronization after a specified number of correct detections (based upon error probability levels) has been attained; or PA1 if a false code is encountered, slip by one bit, and then check the next candidate frame as described above.
This technique is effective, but the expected number of frames requiring examination prior to location of the proper synchronization point is relatively high.